(1) Field of the Invention
This invention relates to an apparatus and a method for inspecting masks which comprise transparent phase shifting mask elements.
(2) Description of the Related Art
Inspection of phase shifting mask elements is a problem because defects are not readily apparent in the transparent phase shifting material.
U.S. Pat. No. 5,270,796 to Tokui et al. describes an apparatus for inspecting a phase shift mask using a phase difference detector and a reference signal. The reference signal is generated from another mask having the same pattern or from CAD data.
U.S. Pat. No. 5,353,116 to Tanigawa et al. describes a defect inspection system which projects two pattern images from two neighboring dies on the phase shift mask. The two images are superimposed and compared.
U.S. Pat. No. 5,379,348 to Watanabe et al. describes a pattern defects inspection system for phase shifting masks.
U.S. Pat. No. 5,446,540 to Lin describes a method of inspecting phase shift masks using phase-error enhancing.
U.S. Pat. No. 5,482,799 to Isao et al. describes a phase shifting mask and a method of manufacturing the phase shifting mask.
U.S. Pat. No. 5,677,092 to Takekuma et al. describes a method of fabricating a phase shifting mask and a method of inspecting the masks.